In recent years, vehicles have been equipped with power steering apparatus for assisting a driver's steering operation. As a power steering apparatus, an electric power steering apparatus (EPS) is widely used, which has a brushless DC motor serving as a drive source. Generally, an EPS incorporates an inverter device for driving a brushless DC motor.
Power for an inverter device is supplied by a battery mounted in a vehicle. The inverter device comprises six semiconductor switching elements (FETs): three on the battery side, and three on the ground side. In order to drive the three FETs on the battery side, voltage higher than the battery voltage has to be supplied to the respective gates of these FETs. For this reason, an EPS includes a gate driver power source separately from the battery.
However, the conventional EPS is not provided with a means for specifying whether abnormality is caused by an FET or by a gate driver in a case where any abnormality occurs in motor control. Therefore, for example, in a case where only one phase of a brushless DC motor is disconnected due to an FET malfunction, assist control has to be stopped even though assist control using the remaining normal phases is possible. In a word, in the conventional EPS, control of switching between assist control and assist stop control is not appropriately exerted depending on a factor of abnormality in motor control.